Fasteners, such as bolts, rivets, split pins and welds, are used in various components in a nuclear power plant. In particular, core barrel bolts are used to fix baffles in place in nuclear reactor pressure vessels. FIG. 1a shows a perspective cross-sectional view of components of an exemplary pressurized water reactor (PWR) 10 that are fixed together by bolts. PWR 10 includes reactor vessel (RV) internals including a core barrel assembly 11. Core barrel assembly 11 includes a cylindrical core barrel 12 and a plurality of baffle plates 14 fixed to an inner circumferential surface 12a of core barrel 12 by a plurality of former plates 16. Baffle plates 14 are situated on top of a lower core plate 18, which is positioned above a lower support plate 20.
FIG. 1b shows an enlarged cross-sectional view of a portion of core barrel assembly 11 in which core barrel 12, baffle plates 14 and former plates 16 are connected together by a plurality of fastened connections using bolts 22, 24, 26. On a left half of FIG. 1b, baffles plates 14 are omitted to more clearly show former plates 16. Baffle plates 14 extend vertically, parallel to a center axis of core barrel 12, while former plates 16 extend horizontally, perpendicular to the center axis of core barrel 12. Baffle plates 14 are fixed together to define a plurality of right angles such that core facing vertically extending surfaces 15a of baffle plates 14 define a fluid flow path in the reactor core, with each baffle plate 14 being arranged perpendicularly to the two adjacent baffle plates 14. Baffle plates 14 may be fixed together by baffle-to-baffle (B-B) bolts 22.
Former plates 16 each include at least two inner edges 17a each for contacting a barrel facing vertically extending surface 15b of one of baffle plates 14. Inner edges 17a are each arranged perpendicular to the two adjacent inner edges 17a. Former plates 16 also each include a rounded outer edge 17b that is shaped to rest flush against inner circumferential surface 12a of core barrel 12. Former plates 16, at outer edges 17b, are fixed to inner circumferential surface 12a of core barrel 12 by core barrel-to-former (CB-F) bolts 24 that pass radially through core barrel 12 and into former plates 16 at outer edges 17b. Baffle plates 14, at barrel facing surfaces 15b, are fixed to inner edges 17a of former plates 16 by baffle-to-former (B-F) bolts 26 that pass radially through vertically extending surfaces 15a, 15b and into former plates 16 at inner edges 17a. These bolted connections are susceptible to degradation mechanisms that result in bolt failure, which is a loss of fastening capacity. Accordingly, the integrity of a structure with these bolted connections needs to be evaluated to account for future bolt failures. A redundant set of fasteners that may need to be assessed for degradation are also used in various other assemblies in nuclear power plants.
The Electric Power Research Institute (EPRI) Materials Reliability Program (MRP) developed inspection and evaluation (I&E) guidelines in MRP-227-A for managing long-term aging of PWR reactor vessel (RV) internal components. The I&E guidelines concentrate on eight aging degradation mechanisms and their aging effects. The I&E guidelines define requirements for inspections that will allow owners of PWRs to demonstrate that the effects of aging degradation are adequately managed for the period of extended operation.
MRP-227-A includes a safety evaluation report (SER) prepared by the U. S. Nuclear Regulatory Commission (NRC). The NRC staff determined whether the guidance contained in the report provided reasonable assurance that the I&E guidelines ensured that the RV internals components will maintain their intended functions during the period of extended operation. From the determination, seven topical report conditions and eight plant-specific applicant/licensee action items (A/LAIs) were contained in the SER to alleviate issues and concerns of the NRC staff. The plant-specific A/LAIs address topics related to the implementation of MRP-227 that could not be effectively addressed on a generic basis in MRP-227.
The sixth A/LAI addresses NRC staff concerns regarding inaccessible components for Babcock & Wilcox (B&W)-designed components. A/LAI 6 lists B&W external baffle-to-baffle bolts, B&W core barrel-to-former bolts, and B&W core barrel assembly internal baffle-to-baffle bolts as inaccessible components that do not need to be inspected under MRP-227 and B&W core barrel assembly internal baffle-to-baffle bolts as accessible components that are non-inspectable using currently available examination techniques. A/LAI then states that, to justify the acceptability of these components for continued operation through the period of extended operation of these components, these components should be evaluated or scheduled for replacement.
One current industry solution involves performing a detailed analysis of the structural integrity with a postulated future bolt failure pattern. If failures are identified by ultrasonic inspection at a given unit, an acceptable operating interval (i.e., re-examination interval) for that unit is defined in WCAP-17096 Rev 2, based on the number of required bolts, which is determined per a detailed analysis, and the number of bolt failures and/or replacements. Such a re-examination interval is based on a deterministic (i.e., a single future failure pattern) evaluation of structural integrity, such as that defined in WCAP-15030-NP-A.